Cryptic speciation in the Warbling Vireo (Vireo gilvus)

Eastern (Vireo gilvus gilvus) and  western (V. g. swainsoni) forms of the Warbling Vireo have essentially allopatric breeding ranges across north-central North America, but come into contact in central Alberta, Canada.  In 1986, Jon Barlow presented preliminary morphological and song evidence suggesting that the Warbling Vireo complex might comprise more than one valid species.  However, to date Barlow’s suggestion is supported by only limited DNA evidence, demonstration of molt and migration differences between the taxa, and anecdotal accounts of differences in song, morphology, plumage, and ecology.  We analyzed variation in both mitochondrial and nuclear DNA in birds from Alberta and surrounding areas to determine the levels of genetic differentiation and  hybridization occurring in the contact zone, and whether the two taxa warrant recognition as separate biological species.  Our analyses reveal that Warbling Vireos in Alberta and the surrounding areas are separated into two well-de...